This invention relates to an integrated high frequency amplifier incorporated with a feedback circuit.
In an integrated circuit (IC), since it is generally difficult to provide for a high frequency high gain amplifier and AC coupling, all stages of the amplifier, that is from the first to the last stages are directly connected. As a consequence, a gain at a low frequency which is not necessary to the inherent purpose of a high frequency amplifier is also obtained at the same time. In such case, variation in the offset presents a problem. For example, in an amplifier having a gain of 60 dB, an offset voltage of 2 mV at the input stage results in a bias variation of 2 V(2 mV.times.1000) at an output terminal which causes saturation of the amplifier in a certain case. A negative feedback is generally used for improving such offset variation.
In an amplifier circuit including a number of gain stages whether the feedback should be applied locally, in other words for individual gain stages or to the whole amplifier circuit or not is determined depending upon the field of use of the integrated circuit. In any case, it is necessary to decrease low frequency gain which is not necessary to the inherent purpose of the high frequency amplifier while at the same time to improve variation caused by the offset.
One example of a prior art negative feedback amplifier generally utilized in an integrated circuit is shown in FIG. 1, and the gain-frequency characteristic of this negative feedback circuit is shown in FIG. 2. As shown in FIG. 1, there are provided a pair of NPN transistors Q1 and Q2 with their emitter electrodes commonly connected. These transistors constitute a differential amplifier for amplifiying a high frequency signal, in which an input signal is applied to the input terminal or base electrode of one transistor and a feedback signal is applied to the input terminal or base of the other transistor. An NPN transistor Q3 is provided for amplifying the output of the transistor Q2. Serially connected feedback resistor Rf and an integrating resistor Ri and capacitor Ci comprise a low pass band type feedback circuit. IN represent an input terminal adapted to receive the input signal and OUT an output terminal.
In this circuit, suppose now that the differential amplifier has a sufficiently high gain without the feedback circuit, then the gain G of this circuit is determined by the following equation: ##EQU1## Thus, within a range of Ri&gt;&gt;1/Ci the gain G would become ##EQU2## showing that the gain is constant irrespective of the frequency.
The gain-frequency characteristic is shown in FIG. 2 in which the ordinate represents the gain and the abscissa represents the frequency. As shown, at frequencies above frequency f2 the gain is constant. The point f2 at which the gain begins to decrease in the low frequency range is shown by the following equation: EQU f2=1/2.pi.Ci.multidot.Ri
The frequency f1 at which the gain becomes substantially unity in the low frequency range is given by EQU f1=1/2.pi.Ci(Ri+Rf)
In a low frequency range in which the offset causes a problem, Ri&lt;&lt;(1/.omega.Ci) and G=1. Since the value of the offset voltage in the input stage appears as it is at the output terminal OUT, the variation caused by the offset described above can be improved and it is also possible to decrease unwanted gain in a low frequency range lower than f2.
The result of experiment made on the circuit shown in FIG. 1 shows that a gain of 12 dB can be obtained in a frequency range of from 5 MHz to 80 MHz. In this case, Rf=1k.OMEGA., Ri=220.OMEGA. and Ci=220 .rho.F are given as illustrative feedback circuit constants.
However, when this circuit is constructed as a monolithic IC, large tip area is required to obtain large capacitance such as 220 .rho.F in the IC, thus increasing the manufacturing cost. For this reason the capacitor Ci was constructed as a component outside of the integrated circuit.
In an integrated circuit having several gain stages each having a construction described above not only the number of the external capacitance increases but also the number of the output terminals increases thus increasing the size of the integrated circuit package.